A workpiece of odd configuration is often difficult to machine, or to perform other like operations upon it, because of the difficulty in holding the workpiece relative to a machining apparatus due to the odd configuration. One solution to this problem is to solidify a meltable matrix material about the workpiece to form a workholder for supporting the workpiece. The workholder then is precisely held relative to a machining apparatus so that the workpiece can be accurately machined. Upon completion of the machining, the matrix material is heated above its melting point, and the workpiece is removed from the molten matrix material.
Conventionally, matrix materials used for fixturing are meltable at relatively low temperatures and include materials such as some metal alloys and organic compounds. The following are brief descriptions of references wherein methods and/or apparatus are used with meltable matrix materials to form workholders for supporting workpieces.
Speyer, U.S. Pat. No. 3,921,343, discloses hot melt compositions including thermoplastic resinoids for fixturing hard-to-hold fragile parts during machining. The resinoid is the reaction product of a methylolurea with a fatty acid.
Lapac et al. U.S. Pat. No. 3,897,535, describes a process for fixturing a workpiece by quenching a liquid resin. The process includes pouring a liquid organic resin to form a workholding material into a fluid tight vessel around a workpiece and quenching the assembly at between 40.degree. F. and 73.degree. F. to solidify the workholding material. Ceramic stones may be heated and placed within the vessel to heat the vessel and workpiece prior to receiving the liquid workholding material. The stones also provide increased holding strength for the workholding material and reduce shrinkage of the material during cooling.
Parsons, U.S. Pat. No. 3,790,152, teaches a chucking apparatus of the meltable matrix type. Molten matrix metal, preferably a eutectic alloy Ostalloy having a melting point of 158.degree. F., is poured into the chucking apparatus and cooled to fixture a workpiece. To prevent the matrix metal from adhering directly to the metal workpiece, the surface of the workpiece may be first coated with a heat curable adhesive, which is cured by the heat of the cooling matrix metal.
Hertner et al, U.S. Pat. No. 995,537, shows a jig for holding articles, or workpieces, during machining. The jig receives a liquid alloy metal which is solidified to fixture a workpiece. Included in the jig are conduits for receiving steam and cold water which serve, respectively, to liquify and solidify the alloy metal.
These methods and apparatus for fixturing a workpiece within a meltable matrix material have a number of disadvantages. First, using meltable matrix materials requires the use of a heating source to liquify the material. Second, the energy supplied to the heating source increases the cost of the fixturing operation. Third, the meltable materials are often very expensive. Therefore, any replacement of lost materials also increases the cost of the fixturing operation.
Fourth, the vessel used to hold the material should be fluid tight during thermal cyling after repeated use. Fifth, the metal alloys or organic materials interact with, and may have deleterious effects on, the surface of the workpieces. This may be especially troublesome if the workpiece is made of a particularly reactive or sensitive alloy. The strength or working life of the workpiece may be reduced as a result of this interaction.
Sixth, it is often difficult to remove all of the meltable matrix material from the machined workpiece. Additional operations may be needed to cleanse the surface of the workpieces. Seventh, the heating and cooling may cause warpage or distortion of the workpiece. Also, the time needed to heat and cool the matrix materials can significantly increase the time necessary to complete the overall machining of a subject workpiece.
Special precautions to guard against health risks may be required. Conventional methods of fixturing can result in workers being burned by hot apparatus or by hot matrix materials. Additionally, the metal alloys or organic materials may be toxic and harmful to human health.
The present invention avoids many of these problems.